KotOR II for sure, I could play KotOR I without any problems, even without any patch...
In KotOR II I just can't get past a god damn turbolift to enter the fuel mining depot during the first mission, game keeps locking up on loading screens or CTD's at random during the game and after days of torture getting the game to actually play with sound....
So my conclusion is: KotOR II is by far the buggiest game I've ever encountered in my entire life, and I'm not exaggerating...